The process of the present invention is particularly applicable but not necessarily restricted to the procedure of binary titanium-molybdenum alloys containing from about 5% up to about 40% molybdenum which are characterized as being substantially homogeneous and devoid of any segregations resulting from undissolved pure molybdenum particles present in the microstructure. Titanium-molybdenum alloys containing molybdenum within a range of about 15 to about 40% are referred to as beta-alloys in that they retain a cubic structure at room temperature. Such alloys containing from about 15 up to about 30% molybdenum are recognized as possessing excellent resistance to corrosion by sodium chloride solutions, as well as exceptional resistance to corrosion by boiling hydrochloric acid and boiling sulfuric acid. The molybdenum constitutent imparts improved strength and enhances the corrosion resistance of such titanium-based alloys. Although these alloys have excellent potential for a number of commercial applications, the production and marketing of titanium-molybdenum alloys have been retarded due to the difficulty encountered in accordance with the prior art practices in obtaining a complete dissolution of the molybdenum in the binary alloy matrix.
It has heretofore been proposed to produce such titanium-molybdenum binary alloys by employing conventional consumable electrode arc melting processes in which the consumable electrode is composed of titanium sponge admixed with particulated molybdenum to produce the desired ratio of the two alloying elements desired in the final binary alloy. A high-energy arc is formed between the end of the consumable electrode and the metal bath within a water-cooled copper crucible or mold. As soon as the titanium metal at the end of the consumable electrode reaches its melting point (1670.degree. C), the titanium becomes liquid and drops into the bath. The molten metal in the bath is only slightly above the melting point of titanium and it is not possible to develop sufficient superheating of either the electrode or the bath to effect a melting of the molybdenum constituent. Accordingly, as the titanium in the electrode melts, the molybdenum particles are released and fall from the electrode as a solid and enter the molten bath in a solid phase. While some dissolution of the molybdenum particle into the molten titanium occurs, the binary alloy formed immediately adjacent to, and surrounding, the molybdenum particle, having a solidus temperature above the melting point of titanium, solidifies, whereby further dissolution of the molybdenum particle ceases. The casting therefore retains undissolved particles of molybdenum metal and/or localized zones of molybdenum contents far above the molybdenum content of the matrix.
In order to overcome this problem, it has been suggested to employ multiple remelting of the ingot produced to effect a further dissolution and diffusion of the molybdenum particles into the titanium matrix. Even after a series of multiple remelting steps, the resultant alloy is still characterized as being of a heterogeneous structure containing undissolved metallic molybdenum particles rendering such ingots unacceptable for the fabrication of mill products, such as rods, bars and tubes for use in corrosion resistant or structural applications.
The process of the present invention overcomes the problems and disadvantages associated with prior art techniques and enables the production of binary alloys, and particularly, titanium-base alloys incorporating about 15 to 30% molybdenum, which are characterized by the fact that the molybdenum is completely dissolved and substantially uniformly distributed throughout the binary alloy matrix, providing an ingot possessed of optimum mechanical properties, enabling the fabrication of mill products possessed of the requisite corrosion resistance and mechanical strength at commercially acceptable costs.